Once you start Qt Designer you should spot a new "Marble Desktop Globe" category in the widget box on the left. You can just use this MarbleWidget like all other widgets by dragging it onto your form and by adjusting the properties of the MarbleWidget.

As a ready-made example you can load the two examples in:

marble/examples/example1.ui
marble/examples/example2.ui

The examples nicely demonstrate how signals and slots work with the MarbleWidget.